1. Field of the Invention:
The present invention relates to an improved process for the catalytic dimerization of alkyl esters of acrylic acid to produce alkyl diesters of dihydromuconic acid.
2. Description of the Prior Art:
In European Patent Application No. 87/420,078.5 is described a process for the catalytic dimerization of a lower alkyl acrylate to produce alkyl diesters of dihydromuconic acid, by the reaction, at a temperature of from 50 to 250.degree. C., of a lower alkyl acrylate in the presence of a catalytic system comprising palladium or a palladium compound, characterized in that the catalytic system is formed from at least:
(a) an unhalogenated source of palladium;
(b) a phosphorus (III) compound of the general formula (I): ##STR1## in which:
R.sub.1, R.sub.2 and R.sub.3 denote, independently, an alkyl, cycloalkyl, aryl, alkoxy, cycloalkoxy or aryloxy radical, with the proviso that one of the radicals R.sub.1, R.sub.2 or R.sub.3, in addition, may be a monovalent radical of the general formula (II): ##STR2## in which:
m is an integer ranging from 1 to 4, inclusive, and R.sub.4 and R.sub.5 denote, independently, an alkyl, cycloalkyl, aryl, alkoxy or aryloxy radical; and
(c) at least one hydracid HY in which the associated anion Y.sup.- is non-coordinating in nature with respect to palladium ions
The diesters thereby produced are intermediates which are useful, in particular in the preparation of alkyl adipates or even adipic acid.
The starting material is a lower alkyl acrylate, namely, one in which the alkyl residue contains from 1 to 8 carbon atoms; the size of the alkyl group is not critical and methyl acrylate and ethyl acrylate are preferably employed because of their greater availability. The alkyl group can contain substituents that do not interfere with the desired reaction.
Naturally, commercial products, not necessarily pure, can be used as the starting material.
The catalytic system according to this particular patent application is formed from at least one unhalogenated source of palladium. This palladium source, the precursor of the catalytically active entity, may be selected from among the various unhalogenated forms of palladium (0) or palladium (II). Among the forms of palladium (II) that are suitable for carrying out such process, exemplary are the salts of organic acids, such as palladium (II) acetate, palladium (II) formate, palladium (II) octanoate and palladium (II) ethylhexanoate; the salts of inorganic acids, such as palladium (II) nitrate; .pi.-allyl complexes of palladium (II), such as (.pi.-allyl)palladium diacetate; and palladium (II) acetyl acetonate.
The use of palladium (II) acetate or palladium (II) acetylacetonate is preferred, in particular because of their greater availability.
Among the sources of palladium (0) suitable for carrying out such process, exemplary are palladium black, palladium deposited on a support such as activated charcoal or silica gel, and complexes of palladium and a trialkyl- or triarylphosphine such as tetrakistriphenylphosphinepalladium.
Known useful such compounds are compounds of palladium (0) and dibenzylideneacetone, which correspond to the general formula (III) below: EQU Pd.sub.x (dba).sub.y (III)
in which:
x is equal to 1 or 2;
dba denotes a dibenzylideneacetone ligand; and
y is equal to 2 or 3, with y necessarily being equal to 3 when x equals 2.
These compounds are readily available. They may be easily prepared by the reduction of palladium chloride in the presence of dibenzylideneacetone (dba) according to any of the procedures described by Y. Ishii et al, Chem. Comm., p. 1065 (1970).
The complexes Pd(dba).sub.2, Pd.sub.2 (dba).sub.3 and Pd(dba).sub.3, and mixtures thereof, may thus be used.
Also in said patent application noted above, it is stated that a concentration of palladium in the reaction medium of at least 0.1 millimole per mole of acrylate appeared to be necessary in order to obtain a sufficient degree of conversion, and no advantage is said to be observed in exceeding the quantity of 3 millimoles of acrylate.
As regards the phosphorus (III) compound, it was specified in the '078.5 patent application that its quantity is generally such that the mole ratio P/Pd ranges from 1 to 15with a mole ratio of from 1 to 3 being preferred.
As regards the hydracid HY, in which the associated anion Y.sup.- is non-coordinating in nature with respect to palladium ions, preferred was tetrafluoroboric acid, and it was stated that a quantity of acid such that the ratio H.sup.+ /Pd ranges from 1 to 30, and preferably from 1 to 10, was required, it being possible to obtain good results with an H.sup.+ /P radio on the order of approximately 2 to 5.
The preparation of the catalytic system at the time of use and its in situ preparation under the dimerization conditions were described, it being possible for the former to entail the prior synthesis of a hydrogenophosphonium salt such as tributylphosphonium tetrafluoroborate.
However, while the basic value of such a process is apparent, its development on an industrial scale prompts at least one of the following problems:
(1) insufficient activity of the catalytic system;
(2) lack of selectivity of the catalytic system which results, in particular, in the production of a significant proportion of heavy products; and
(3) lack of stability of the palladium, which is capable of being deposited onto the walls of the reactor and which is capable of inducing substantial variations in activity with the passage of time.
These various problems become a matter of more particular concern when it is desired to carry out the process under conditions of temperature and palladium concentration which are compatible with industrial requirements, that is to say, a temperature of from 70.degree. to 150.degree. C. and a palladium concentration of from 0.1 to 3 mmol of palladium per mole of alkyl acrylate.
Cf. U.S. Pat. Nos. 3,792,101, 4,594,447 and 4,638,084, and FR-A-2,524,341.